Method for cooling work pieces especially shape-rolled products from rail steel

ABSTRACT

The invention relates to a method for producing especially shape-rolled products from rail steels that have a finely perlitic and/or ferritic/perlitic structure after cooling. The inventive method is characterized by guiding the workpiece through a cooling stretch that is composed of individual independent cooling modules ( 2   a - e ) having independently adjustable cooling parameters. Between said cooling modules ( 2   a - e ) intermediate zones ( 5   a - e ) are provided for relieving the stress of the structure comprising means for determining the actual temperature (T ACTUAL ) of the respective work piece in said intermediate zones ( 5   a - e ) Depending on the respective actual temperature values (T ACTUAL ) of the work piece in an intermediate zone ( 5   a - e ) the specific cooling parameters, especially the cooling intensity, of the respective subsequent cooling module ( 2   b - e ) are controlled in order to safeguard a defined temperature of the work piece during the entire passage through the cooling stretch ( 1 ), the defined temperature (T DESIRED ) of the work piece being above a critical temperature at which the bainitic portions of the structure are formed.

[0001] The invention relates to a method of cooling workpieces,especially for the cooling of rolled products and here to a method ofcooling shape-rolled products [rolled structural shapes] of rail steelswith a fine perlitic or a ferritic/perlitic structure, whereby the hotworkpiece, that is a workpiece with an authentic structure, is passedthrough a cooling stretch with an inlet region and an outlet region andis subjected to a cooling process and as a result a transformation iscarried out into a perlitic or ferritic/perlitic structure.

[0002] Rail steels are significant for the production of rails as wellas of their connection elements and their fastening elements. Thevertical and lateral forces which are applied by the wheel to the rail,like normal forces, traveling forces, acceleration forces and brakingforces, give rise in the regions in which they are directly effective toextremely high dynamic stresses and, as a rule, to a plastic deformationof the steel. As a result of these loads, wear effects arise in the formof ablation of material, friction wear, material breakage, localworkpiece fatigue or cracking. An improvement in the resistance of arail to these wear effects can be achieved by increasing its elasticlimit and tensile strength as well as its fatigue limit in combinationwith the provision of the finest possible striated perlitic structure.

[0003] Under normal cooling conditions using a cooling bed in accordancewith the state of the art, rail steels undergo a transformation to aperlitic structure. In this manner rail steels with a ferritic/perliticstructure can reach tensile strength values in a range of 700 to 900N/mm² while steels with a purely perlitic structure can achieve tensilestrength values in excess of 900 N/mm². The significant properties ofthe rail steels are determined by the proportion of the structureconstituted by ferrite/perlite as well as by its morphologicalstructure. Both in the case of ferritic/perlitic steels and in the caseof perlitic steels, the lamellae spacing plays a role.

[0004] The invention sets out as its object to provide a cooling processfor the production of workpieces, especially shape rolled products fromrail steel with improved properties and a fine striated perlite orferrite/perlite structure.

[0005] According to the invention it is proposed that the workpiece,which is at its (rolling) heat, for example a rolled or optionally anextruded structural-shape product, is passed through a cooling stretchwhich is assembled from individual/independent cooling modules withindependently adjustable cooling parameters, whereby between the coolingmodules there are intermediate zones for thermal equalization or thermalstress relief with means for an actual temperature determination for therespective workpiece in this intermediate zone and whereby, independence upon the respective actual temperature value in theintermediate zone or in one of the intermediate zones, the specificcooling parameters, especially the cooling intensity, of the respectivesubsequent cooling module, are controlled in order to maintain a defined(surface) temperature during the entire passage along the coolingstretch, whereby the defined temperatures of the workpieces respectivelyeach lie above a critical temperature at which the bainitic portions ofthe structure are formed.

[0006] The basic concept is, therefore, the control of the cooling of aworkpiece of rail steel in a cooling stretch under the condition thatthe surface temperature of the workpiece of rail steel is so cooled thatthe desired perlitic or ferritic or perlitic/ferlitic structure isestablished, whereby in the stress relief phases there is both acontinuous monitoring of the temperature characteristics in preferablyeach intermediate zone and optionally regulation of the coolingparameters of the individual cooling modules to ensure that thetemperature does not fall below a critical temperature so that the undercooling cannot result in a bainitic transformation which can give riseto undesirable bainitic components in the structure.

[0007] The cooling process is carried out by the workpiece traversingcooling modules in individual cooling process steps and in dependenceupon the conditions in the intermediate zones for stress relief of thestructure in timed phases which can include reheating and/or timedphases in which thermal conditions are maintained and/or timed phasesprovide a slower cooling, taken together. The workpiece in allintermediate zones can be subjected to the same stress relief phase orcan be subjected to different timed phases of stress relief in thevarious intermediate zones. The reheating can be effected either fromresidual heat from the interior of the workpiece and still presenttherein and/or from the supply of heat to the workpiece from theexterior. In this manner a somewhat sawtooth cooling pattern isestablished which has been found to be particularly effective inestablishing the desired final internal structure of the workpiece andthus the mechanical properties thereof. The bainitic formation iscounteracted in that the parameters of the cooling stretch are so setthat at no point in time during the cooling process can bainiteformation take place.

[0008] It is also provided by the invention that the intermediate zonesare utilized for a thermal equalization over the workpiece, especiallyrolled products, or for the cooling thereof at slow cooling speeds.

[0009] Preferably the respective measured actual temperature value ineach of the intermediate zones is utilized to control the specificcooling parameters of the respective subsequent cooling zone andsimultaneously the cooling parameters of the respective precedingcooling modules. This means that the workpiece or rolled product to theextent that it deviates from a predetermined setpoint temperature at acertain point in time or in a particular intermediate zone, is broughtback to the setpoint temperature by a specific change in the coolingparameters of the subsequent cooling module and at the same time thepreceding cooling module is adjusted for the next workpiece to follow inthe sequence.

[0010] Advantageously, the surface temperature of the workpiece at theend of the intermediate zone, i.e. following the end of the region inwhich structure destressing occurs, is measured. The temperaturemeasurement in the intermediate zone can also be used for qualitymonitoring.

[0011] According to a preferred embodiment, the surface temperaturemeasurement is effected by an optical and contactless measuring device,that is by means of a pyrometer.

[0012] The control of the cooling parameters and here especially thecooling intensity is effected preferably by means of control of thepressure with which the cooling medium is directed onto the surface ofthe workpiece and/or by means of regulation or regulated adjustment ofthe temperature of the cooling medium and/or by means of controlledadjustment of the volume rate of flow of the cooling medium by selectionof the cooling nozzle geometry. As the cooling medium, preferablycooling water is used.

[0013] The pressure control is effected preferably by means of apressure control valve in the inlet to the nozzles and which may bearranged on the cooling beams. The cooling intensity is alsocontrollable by utilizing different numbers of nozzles per cooling beamor cooling beam arrangements.

[0014] According to an especially preferred embodiment of thetemperature control of the cooling medium it is proposed that thecooling medium, that is especially cooling water, before its impingementupon the workpiece surface, is preheated at least to the extent thatundershooting of the Leidenfrost temperature does not occur or is verygreatly delayed.

[0015] The Leidenfrost phenomenon is a nonwetting property of a liquidwhen the temperature of the contacted body lies above the boilingtemperature of the liquid. Water, for example, is protected by a gasskin of vaporized water from further evaporation and thus loses for acertain time its cooling effectiveness. By preheating the cooling waterit is possible to influence the Leidenfrost temperature. The Leidenfrosttemperature increases with increasing cooling water temperature at theinlet and the cooling effect is weakened. So that an undershoot of theLeidenfrost temperature will not occur or will be delayed significantly,it is proposed to preheat the cooling water. This offers the possibilityof weakening the cooling and making it more reproducible.

[0016] According to a preferred method step, the temperature of theworkpiece before or upon entry into the cooling stretch is measured andbased upon this temperature measurement the cooling parameters of thecooling line are preset especially in terms of the adjustment of thepressure with which the cooling medium is directed upon the workpiecesurface.

[0017] Further details and advantages of the invention will be obtainedfrom the dependent claims and the following description in which theembodiments of the invention illustrated in the FIGURES are described ingreater detail.

[0018] Apart from the previously described combinations of features,features of the invention can be taken alone or can be consideredsignificant to others in other combinations.

[0019] The drawing shows:

[0020]FIG. 1 is a schematic overview of a coding stretch in which themethod of the invention is carried out;

[0021]FIG. 2 is a temperature-time diagram with the cooling curves offive measurement points in or on the railhead of a usual rail steel withabout 0.8% C and 1.0% Mn which is subjected according to the inventionto such a cooling pattern in a cooling stretch according to theinvention in which the bainitic temperature is not undershot;

[0022]FIG. 3 for comparison is a temperature-time diagram of the fivecooling curves of an unregulated course of cooling wherein the bainitetemperature is undershot.

[0023] The cooling stretch 1 illustrated in FIG. 1 is connected to astructural shape rolling line (not illustrated), for example, a rollingline for rail structural shapes of rail steels. The cooling stretch 1 iscomprised, in the illustrated embodiment, of five cool modules 2 a-e,but is not however limited to this number of cooling modules. Theindividual cooling modules 2 a-2 e are for example so constructed thatthey encompass one or more cooling beams or cooling nozzle arrangements.The pressure with which the cooling water emerges from the individualnozzles is adjustable by means of the respective pressure control valves2 a- e. The actual pressure is measured by means of the pressuremeasuring devices 4 a-e. Between the individual cooling modules 2 a-e,intermediate zones 5 a-e are arranged. At each end of an intermediatezone 5 a-e a pyrometer 6 a-e is located for the contactless opticalmeasurement of the surface temperature of the rolled product found inthis intermediate zone, whereby in the case of a rail structure shape,the surface temperature at the rail head is measured.

[0024] Upstream of the first cooling module 2a at the inlet region orbeginning 12 of the cooling stretch 1 an additional pyrometer 6 f isdisposed. The individual pyrometers 6 a-f are connected by means ofsignal connectors 7 a-g with a computer unit 8. The computer unit 8 isconnected by corresponding control conductors 9 a-e to the individualcontrol valves 2 a-e for the cooling nozzles to vary the settings ofthese control values. The cooling medium, especially cooling water (cw)is supplied by a common feed pipe 10 with branches 10 a-e connected tothe individual cooling modules 2 a-e.

[0025] For regulating the pressure values, there is in addition acontrol circuit of the pressure measurement devices 4 a-e for thecomputer 8 (signal conductors 11 a-e). In the following, the process isdescribed. Prior to entry of the rolled structure shape of steel,preferably a rail, into the cooling stretch by means of the firstpyrometer 6 f, for example a two color pyrometer, an actual surfacetemperature value is taken. This first surface temperature value is fedto the computer unit 8 which has already been provided with a presettingin response to this individual value. For the individual control valuesfor the setting of the cooling water pressure as well as the coolingwater temperature. After the workpiece has traversed the first coolingmodule 2 a it enters the first intermediate zone 5 a in which a reliefor destressing phase for the structure is effected. At the end of thefirst intermediate zone 5 a, by means of a second pyrometer 6 a, forexample a two color pyrometer, a further surface temperature measurement(T_(ACT)) This actual value is transferred to the computer unit 8 overthe signal lines 7 a and 7 g and then a difference calculation iscarried out between a setpoint value T_(SET) and the actual value(T_(ACT)). The setpoint value always lies immediately above aworkpiece-specific temperature at which bainite formation can arise. Thesetpoint values are alloy—specific and can be obtained by experiments. Adetermining factor for this critical temperature below which the railsteel should not be cooled, is about 450 to 500° C.

[0026] To the extent that there is a difference between the actual valueand the setpoint value, the subsequent or a plurality of subsequentcooling modules have their cooling parameters adjusted, here by varyingthe pressure control valves 2 a-3 e which regulate the pressure of thecool water directly onto the workpieces. The regulation of the pressurevalues in dependence upon a measure of the actual pressure value iscarried out continuously.

[0027] The described control is repeated in dependence upon therespective temperature values detected in each further intermediatezone. Preferably not only is each subsequent cooling module adjusted butalso the preceding cooling module is adjusted for each measured valuewhich then affects the subsequent rolled workpiece to be cooled.

[0028]FIGS. 2 and 3 show with the aid of temperature-time diagrams thecooling curves for the rail heads of a material with 0.8% carbon withcontrol and without control. The designation C80W60 or C80W65 makesclear that the cooling speed in the core of the rail head (for example arail shape in accordance with AREA 136 [Standard of American RailwayEngineering Association] is significantly higher than in the boundaryand that in the core transformation of austenite to perlite orferrite-perlite occurs at elevated temperatures.

[0029] The temperature course over time was taken for five differentmeasurement points at the rail head At 1 the measurement point was inthe core of the rail head. 2 was a measurement point which was located 5mm below the surface 3 was a measurement point which was located 5 mmbelow a lateral surface. 4 was a measurement point on the lateralsurface. 5 was a measurement point on the head surface. It can be seenthat at no time at any measurement point did the structure of the railhead suffer an undercooling that could have given rise to a bainitestructure.

[0030] The simulated cool stretch had five modules which wereindividually controllable. The individual cooling curves are illustratedin FIG. 2 and in no case was the critical temperature at which bainiteformation could set in, undershot. On the cooling curves 4 and 5 whichindicate the cooling at the surface of the rail head, the sawtoothcooling pattern is clearly shown and involved reheating in theintermediate or equalization zones.

[0031]FIG. 3 shows by comparison a cooling stretch with five coolingmodules which are not individually controllable so that the bainitetemperature can be undershot in the regions close to the surface (curves4 and 5) of the rail head.

[0032] With the method proposed, a cooling of rail steels from therolling heat can be carried out to yield a fine perlitic orferlitic/perlitic structure without the mechanical properties andespecially the wear properties being negatively affected by bainiticcomponents.

1. A method of cooling workpieces, especially structural shape rolledproducts, of rail steels with a fine perlitic or ferritic orperlitic/ferritic structure, whereby the hot workpiece is guided througha cooling stretch (1) with an inlet region (12) and an outlet region andis subjected to a cooling process and whereby a transformation into aperlitic or ferritic/perlitic structure is effected, characterized inthat the workpiece is passed through a cooling stretch which iscomprised of individual independent cooling modules (2 a-e) withindependently adjustable cooling parameters, whereby between the coolingmodules (2 a-e) intermediate zones (5 a-e) for structure destressing orstress relief are provided with means for actual temperature measurement(T_(ACT)) of the respective workpieces in these intermediate zones (5a-e) and whereby in dependence on the respective actual temperaturevalue (T_(ACT)) of the workpiece in an intermediate zone (5 a-e) thespecific cooling parameters, especially the cooling intensity), of atleast the respective subsequent cooling module (2 b-e) are controlled toensure a defined temperature of the workpiece during the completepassage through the cooling stretch (1), whereby the defined temperature(T_(SET)) of the workpiece respectively lies above a criticaltemperature at which bainitic structure components are formed.
 2. Themethod of cooling according to claim 1 characterized in that the coolingprocess is carried out in dependence upon the cooling module traversedin individual cooling process steps and in dependence upon theintermediate zones traversed for structure stress relief in timed phasesof reheating and/or timed phases of retention of heat and/or in timedphases of slow cooling in combination.
 3. A method of cooling accordingto claim 1 characterized in that in dependence upon the respectivemeasured actual temperature value (T_(ACT)) of one or any intermediatezone (5 a-e) the specific cooling parameters of the respectivesubsequent cooling module (2 b-e) and simultaneously the cool parametersof a preceding cooling module (2 a-d) are controlled.
 4. A method ofcooling according to claim 1 characterized in that the surfacetemperature of the workpiece at the end of the intermediate zone (5 a-e)is measured.
 5. A method of cooling according to claim 1 characterizedin that the actual temperature measurement (T_(ACT)) is carried out bymeans of an optical and contactless measurement.
 6. The method ofcooling according to claim 1 characterized in that the control of thecooling parameters, especially the cooling intensity is achieved bymeans of pressure control and/or temperature control of the coolingmedium.
 7. A process for cooling according to claim 1 characterized inthat the cooling medium especially cooling water before impingement uponthe workpiece surfaces is so preheated that an undershoot of theLeidenfrost temperature does not occur or occurs later than withnonpreheated cooling medium.
 8. A method of cooling according to claim 1characterized in that the temperature of the workpiece before entry orupon entry into the cooling stretch (1) is measured and this temperaturevalue is used for presetting the cooling parameters of the individualcooling modules.